Monomer can often be recovered from polymer waste by depolymerisation. In particular, PMMA, i.e. poly(methyl methacrylate), gives methyl methacrylate at elevated temperature. For many years, PMMA has been successfully depolymerised by contact with molten lead at about 500.degree. C.; the monomer MMA can be obtained in a purity of more than 98% (here and throughout, percentages are given by weight unless otherwise specified).
Although this process gives MMA of high purity, the use of lead is undesirable, from an environmental viewpoint. Further, small amounts of lead as a contaminant are extremely difficult to remove from the monomer, reducing the range of commercial products for which the recovered monomer is acceptable. Higher purity would be desirable, and in particular substantial freedom from byproducts such as methyl isobutyrate which is nonpolymerisable, smelly and difficult to separate from MMA.
Kaminsky et al, J. Anal. App. Pyrolysis 19 (1991) 311-318, disclose the recovery of MMA by pyrolysing PMMA in an indirectly-heated fluidised-bed process. At 450.degree. C., more than 97% of the PMMA was recovered as monomer.